The principle governing the operation of most current magnetic read heads is the change of resistivity of certain materials in the presence of a magnetic field (magneto-resistance or MR). Magneto-resistance can be significantly increased by means of a structure known as a spin valve or SV. The resulting increase (known as Giant Magneto-Resistance or GMR) derives from the fact that electrons in a magnetized solid are subject to significantly less scattering by the lattice when their own magnetization vectors (due to spin) are parallel (as opposed to anti-parallel) to the direction of magnetization of their environment.
The key elements of a spin valve are shown in FIG. 1. They are capping layer 19, conducting layer 18, low coercivity (free) ferromagnetic layer 17, non-magnetic spacer layer 16, and magnetically pinned ferromagnetic layer 15. In earlier designs layer 15 would have been in direct contact with antiferromagnetic (AFM) pinning layer 12 but, more recently, it is made to be part of a synthetic antiferromagnet. This is formed by sandwiching antiferromagnetic coupling layer 14 between it and another ferromagnetic layer 13 whose direction of magnetization is antiparallel to it. This results in an increase in the size of the pinning field so that a more stable pinned layer is obtained. Hence the description of the device as being synthetically pinned. It is convenient to refer to layers 15 and 13 as AP1 and AP2, respectively (AP=antiparallel). The structure is completed by the presence of seed layer 11 which serves to enhance the magnetic properties of AFM layer 12.
When the free layer is exposed to an external magnetic field, the direction of its magnetization is free to rotate according to the direction of the external field. After the external field is removed, the magnetization of the free layer will stay at a direction, which is dictated by the minimum energy state, determined by the crystalline and shape anisotropy, current field, coupling field and demagnetization field. If the direction of the pinned field is parallel to the free layer, electrons passing between the free and pinned layers, suffer less scattering. Thus, the resistance at this state is lower. If, however, the magnetization of the pinned layer is anti-parallel to that of the free layer, electrons moving from one layer into the other will suffer more scattering so the resistance of the structure will increase. The change in resistance of a spin valve is typically 8-20%.
Most GMR devices have been designed so as to measure the resistance of the free layer for current flowing parallel to the film's plane. However, as the quest for ever greater densities continues, devices that measure current flowing perpendicular to the plane (CPP) are also being made. For devices depending on in-plane current, the signal strength is diluted by parallel currents flowing through the other layers of the GMR stack, so these layers should have resistivities as high as possible while the resistance of the leads into and out of the device need not be particularly low. By contrast, in a CPP device, the resistivity of both the leads and the other GMR stack layers dominate and should be as low as possible.
In current synthetically pinned CPP SV designs, AP2 always makes a negative contribution to the device's GMR since its magnetization direction must be anti-parallel to the pinned layer (AP1). The present invention discloses how this negative contribution may be minimized, whereby the CPP GMR as well as dRA can be greatly enhanced.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 5,668,688 Dykes et al. show a CPP SV process. Lederman et al., in U.S. Pat. No. 5,627,704, show a GMR CCP transducer while Lubitz et al. discuss the CPP GMR in U.S. Pat. No. 6,171,693 B1. U.S. Pat. No. 6,278,589 B1 (Gill) shows a dual GMR having a single AFM layer and, in U.S. Pat. No. 6,295,187 B1, Pinarbasi shows a related SV.